The present invention relates to a process for determination of the friction/slip characteristics of a road vehicle tires, in which the vehicle which is equipped with an antilock system configured on the basis of an individual wheel control, according to which, in the traction mode of the vehicle, the course of the respective tire characteristic in the entire .mu./.lambda. field is concluded from pairs of measured values of the slip and of the coefficient of friction utilized at a given slip.
German Patent Application P 41 02 301.3 describes a process which in the traction mode of the vehicle, the tire characteristics of the driven vehicle wheels are determined and, on account of the equivalence of drive slip and brake slip, the thus determined tire characteristics are also used for the determination of brake slip thresholds which when exceeded cause the antilock system of the vehicle to respond. The maximum utilizable coefficients of friction (the .mu.-maximum of the respective tire characteristic are concluded from the spin behavior of the driven vehicle wheels, the speed of rotation of which drastically increases when a slip value corresponding to the maximum of the characteristic is exceeded. A precise determination of the tire characteristics in the braking mode is not possible in accordance with this known process.
Further, WO 85/02592 (PCT/EP84/00402) discloses a process for the determination of an optimal slip value .lambda. at at least one wheel of a vehicle for controlling the brake force with the use of the wheel speed V.sub.R and of the vehicle speed V.sub.F of at least approximate signals. During the journey, the slip .lambda. is varied by altering the basic pressure P.sub.B and at individual instants of measurement (k, k+1, . . .) signal value combinations of the wheel speed V.sub.R * (K), of the vehicle speed V.sub.F * (K), of the brake pressure P.sub.B * (K) and of the normal reaction force F.sub.A * (K) effective at the wheel are determined; following the availability of a plurality of such signal value combinations, determined respectively one following the other by a measurement time interval .DELTA.t, for various values of the slip .lambda., a function V.sub.R (K+1)=f is determined. This function represents the solution of a system of equations with unknown coefficients .alpha..sub.n, the coefficients of which can be determined with the aid of this system of equations. From these coefficients .alpha..sub.n there are then determined coefficients .alpha..sub.n, which are coefficients of a general description of the .mu. slip curve .mu.=f(V.sub.R,V.sub.F), from the course of which the position of the optimal slip value .DELTA..sub.opt is then determined.
This known process requires, during a braking, measurement of parameters which are fundamentally affected by a high error. For example, the wheel normal reaction force F.sub.A, which can be "measured" only indirectly or, in the case of a more direct measurement (e.g. with the aid of strain gauges) by which the spring strain of wheel springs can be picked up, is exposed to considerable disturbance variables as a result of the resilient spring movements. Also the value of the vehicle speed V.sub.F is determinable only very imprecisely, during a braking in which all vehicle wheels are affected by a brake slip, this being at all events the case where the vehicle speed is obtained from an averageing of individual wheel speeds which are picked up by wheel speed-of-rotation sensors individually allocated to the vehicle wheels. Consequently, the known process, which requires the performance of a multiplicity of computation steps, converges only very slowly. That is, a reasonably reliable course--one which is close to reality --of the thus determinable .mu. slip curves can be achieved only after a multiplicity of brakings, but this multiplicity is not carried out in the course of a normal journey.
Accordingly, an object of the present invention is to provide an improved process which permits, in the braking mode of the vehicle, within a short period of time, to a large extent precise determination of the tire characteristic, a knowledge of which is a prerequisite for a slip control to maintain the dynamic stability of a vehicle within the widest possible range, as well as to an improved slip control system with which the process can be carried out.
This object has been achieved by a process in which (d) in an initial phase of a controlled braking linked with moderate vehicle retardation, using brake pressure on only a wheel brake of an individual vehicle wheel and dimensioning the brake pressure in an initial rising phase so that increasing initial retardation in accordance with the driver's wish is at least approximately obtained; (e) as soon as, as a result of just the braking of the test wheel, at least one of the vehicle retardation, z, no longer increases and the braked vehicle wheel is retarded more strongly than corresponds to a predetermined threshold value, interrupting the test braking and continuing the braking by pressure action on other vehicle wheels; and (f) carrying out test braking phases according to steps (d) and (e) in cyclic sequence for all remaining vehicle wheels and by a slip control system in which the front wheel brakes (11, 12) connected to a primary output pressure space (56) of the tandem master cylinder (16') and the rear wheel brakes (13, 14) are connected to a secondary output pressure space (57) of the tandem master cylinder (16)'), a first bore stage (63) of a housing (64) of the tandem master cylinder (16'), forming a housing-defined boundary of the secondary output pressure space (57), within which first bore state (63) a secondary piston (59) with an outer piston flange (74) delimits the secondary output pressure space (57) in relation to a pressureless downstream chamber (82), an inner piston flange (73) movably sealing off the downstream chamber (82) in relation to an inner portion of the first bore stage (63) an intermediate wall (68) of the housing (64) delimiting the bore stage (63) relative to a central bore stage (66) which extends from a third bore stage (61) receiving a primary piston (58), a cross-sectional area (A.sub.1) of the third bore stage (61) is greater than a cross-sectional area (A.sub.3) of the central bore stage (66), an actuating piston (69) being displaceably guided in the central bore stage (66) in a pressure tight manner, the actuating piston (69) forming an axially movable, inner boundary of the primary output pressure space (56) relative to a pressureless downstream chamber (86) in the central bore stage (66) which is bounded by the intermediate wall (68), the actuating piston (69) being provided with a thrust rod (71) configured to axially penetrate the central downstream chamber (86) and to pass displaceably in a pressure tight manner through a central bore (72) of the intermediate wall (68) and being axially supported on an inner piston flange (73) of the secondary piston (59), an annular space (96) which is bounded in an axially movable manner by the inner piston flange (73) of the secondary piston (59) and by the intermediate wall (68), centrally penetrated by the thrust rod (71) and is connected to the pressure output (42) of the controllable pressure source (40).
According to the present invention, in an initial phase of a controlled braking linked with moderate vehicle retardation, the wheel brake of only a single vehicle wheel is acted upon by brake pressure and this brake pressure is rapidly increased such that a development of the vehicle retardation which is in accordance with the driver's wish is obtained. In this initial test phase, the absolute brake slip is continuously measured, and is made possible with very great precision because at least the unbraked vehicle wheels roll freely and thereby very precise information can be obtained on the vehicle speed, a precise knowledge of which is an essential prerequisite for the precise determination of the slip. The vehicle retardation can likewise be determined very precisely by reference to the wheel speeds of rotation of the non-retarded vehicle wheels. The vehicle wheel subjected to the braking can be braked to very close to the "lock limit", since the freely rolling vehicle wheels provide the vehicle with sufficient lateral guidance force in order to guarantee the required stability. The test braking is interrupted as soon as, as a result of just the braking of the test wheel, the vehicle retardation z increases no further and/or the braked vehicle wheel is retarded more strongly than corresponds to a predetermined threshold value. By cyclic performance of these measures, the applicable tire characteristic is determined for all vehicle wheels and can be updated repeatedly, depending upon the duration of the journey or respectively the number of brakings executed during a journey. Thus, changes of the tire characteristics can also be picked up and then utilized for a more situationally realistic setting of response thresholds, e.g. for an antilock system.
Since lower vehicle retardations can be achieved using the rear wheels than using the front wheels, it is advantageous if the test braking phases are carried cut in the "sequence" the test phase is carried out on a rear wheel when a vehicle driver seeks braking with moderate vehicle retardation between 0.1 g and 0.2 g, and on a front wheel when the vehicle driver seeks a somewhat higher vehicle retardation between 0.2 g and 0.4 g or two diagonally opposite vehicle wheels are braked during the test braking phase if the driver seeks a braking retardation of more than 0.4 g or the test braking phase is carried out first on the rear wheel and thereafter on the front wheel.
Furthermore, it is advantageous if, in the case of a performance of the test braking phase on a driven vehicle wheel, the drive train of the vehicle is decoupled from the driven vehicle wheel, in order that a retroaction or reaction of the drive train on the braked vehicle wheel should be excluded.
The driver's wish of "controlled braking" can be recognized alternately or in combination in that the driver actuates the brake pedal of the braking system with a force which is smaller than a predetermined threshold value or in that the vehicle retardation is kept smaller than a settably predetermined threshold value or very precisely also in that the brake pressure which the driver feeds into the braking system by actuating the braking device is smaller than a threshold value.
In place of a tabular input of measured friction/slip value pairs into a memory of an electronic control unit, it is advantageous to generate continuously the tire characteristics determined in accordance with the process according to the present invention such that for the friction coefficient/slip interrelationship which is determined with reference to at least slip and retardation and which is applicable to a respective vehicle wheel, an algorithm is obtained by interpolation or matching of an approximation relation which can be evaluated by an electronic control unit (50) to characteristic base points of stored .mu..sub.B /.lambda..sub.B value pairs obtained by measurement and stored for a continuous processing of measured .lambda..sub.B data in units of friction coefficient currently utilized and determination of the currently utilized friction coefficient, .mu..sub.B, occurs by evaluating the relation EQU .mu..sub.B =C.sub.1 (1-e.sup.-C2.lambda.s)-C.sub.3 .lambda..sub.B
in which C.sub.1, C.sub.2 and C.sub.3 are constants obtained by matching the relation to measured .lambda..sub.B and .mu..sub.B values and are stored as parameters of the relation used for the continuous evaluation and continuously updated.
With respect to a slip control system suitable for carrying out and applying the above-described process according to the invention, a constructionally simple, space saving and especially functionally reliable configuration of a braking device provided within the context of the braking system of the vehicle and particularly advantageous from the viewpoint of safety is specified by the features of one in which the front wheel brakes (11, 12) connected to a primary output pressure space (56) of the tandem master cylinder (16') and the rear wheel brakes (13, 14) are connected to a secondary output pressure space (57) of the tandem master cylinder (16)'), a first bore stage (63) of a housing (64) of the tandem master cylinder (16'), forming a housing-defined boundary of the secondary output pressure space (57), within which first bore state (63) a secondary piston (59) with an outer piston flange (74) delimits the secondary output pressure space (57) in relation to a pressureless downstream chamber (82), an inner piston flange (73) movably sealing off the downstream chamber (82) in relation to an inner portion of the first bore stage (63) an intermediate wall (68) of the housing (64) delimiting the bore stage (63) relative to a central bore stage (66) which extends from a third bore stage (61) receiving a primary piston (58), a cross-sectional area (A.sub.1) of the third bore stage (61) is greater than a cross-sectional area (A.sub.3) of the central bore stage (66), an actuating piston (69) being displaceably guided in the central bore stage (66) in a pressure tight manner, the actuating piston (69) forming an axially movable, inner boundary of the primary output pressure space (56) relative to a pressureless downstream chamber (86) in the central bore stage (66) which is bounded by the intermediate wall (68), the actuating piston (69) being provided with a thrust rod (71) configured to axially penetrate the central downstream chamber (86) and to pass displaceably in a pressure tight manner through a central bore (72) of the intermediate wall (68) and being axially supported on an inner piston flange (73) of the secondary piston (59), an annular space (96) which is bounded in an axially movable manner by the inner piston flange (73) of the secondary piston (59) and by the intermediate wall (68), centrally penetrated by the thrust rod (71) and is connected to the pressure output (42) of the controllable pressure source (40), an end portion (71') of the thrust rod (71) engages at the secondary piston (59), and is operatively arranged in a blind bore (88) of the secondary piston (59) displaceably in pressure tight manner relative thereto and is supportable on a base (91) of the blind bore (88) and the inner piston flange (73) of the secondary piston (59) has a compensating bore (92) opening centrally in the blind bore (88) at the base (91) and connects the blind bore (88) with the downstream chamber (82), and in which braking device closed braking circuits can be utilized.
By way of a secondary cylinder which is connected between the output pressure space, allocated to the front axle braking circuit of the braking system of the vehicle, of the tandem master cylinder and the main brake line, leading onto the front wheel brakes, of the front axle braking circuit and which acts as pressure converter and which for its part has a control pressure space, into which a controllable output pressure of the control pressure source can be coupled, which output pressure can be additively superposed upon the output pressure which can be generated in the output pressure space of this secondary cylinder just by actuation of the master cylinder, additional variability with respect to the brake force distribution control is achieved. Here, it is advantageous if a specifically associated control pressure output of the control pressure source is allocated to the secondary cylinder.
A pressure sensor can be provided to generate an electrical output signal which is characteristic of the pressure in the tandem master cylinder, such that the driver's intent existing with respect to the desired vehicle retardation is recognizable in a simple manner.
Pressure sensors can also be provided to generate electrical output signals which are characteristic of the control pressure made available at the respective pressure output of the control pressure source or electrical output signals which are characteristic of the brake pressures coupled into the individual wheel brakes. These electrical output signals are fed as information inputs to the electronic control unit and permit an accurately metered pressure allocation to the individual wheel brakes while maintaining optimal dynamic stability of the vehicle.
Both for the recognition of situations of travel on a bend and also for the control of the brake force distribution to the individual vehicle wheels in the sense, for example, of a uniform utilization of friction on all vehicle wheels, it is advantageous if the vehicle is equipped with a transverse acceleration sensor and/or a yaw angle sensor to generate electrical output signals which can be processed by the electronic control unit of the slip control system.
To recognize the driver's intent of "controlled braking" or "full braking", it is also advantageous if a force sensor is provided to generate an electrical output signal characteristic of the force with which the driver actuates the brake pedal of the braking system and by which, if the signal level exceeds a threshold value, the performance of a test braking phase is interrupted or the initiation thereof is prevented from the outset.